One embodiment of the present invention relates generally to the electrostatic painting of plastics. Although the invention is described below with relation to the electrostatic spray painting of plastic car body panels, it should be understood that the invention has further uses which will become more apparent from the description to follow.
In a typical electrostatic spray painting process, the spray head is maintained at a high voltage (50-140 KV) while the object being sprayed (the substrate) is electrically grounded. When a metal substrate is painted, it is relatively simple to maintain the metal at ground potential. In the electrostatic painting process, particles (paint droplets) are charged by an electrode in the spray head, and a charged spray cloud from the spray head is attracted to the metal surface by the high voltage difference. This process greatly reduces over-spray and produces a high quality surface on the painted metal parts. For these reasons, and others, electrostatic spray painting techniques have been used for many years in the automotive industry for spray painting exterior body panels made of sheet metal.
In recent years, the automotive industry has increased its use of plastic materials for exterior car body panels and trim parts. The predominant reasons are weight-reduction and the fact that car builders have had available more sophisticated high impact strength plastics such as polycarbonates. To a large extent, the future success of plastics for large car body panels will depend on their ability to be painted "on-line" in the assembly plant with a class "A" quality appearance similar to painted metal car body panels. Electrostatic spray painting of plastic car body panels has been used for years. However, difficulties arise when using electrostatic spray techniques for painting plastic substrates. The problem is particularly difficult when the objective is to paint plastics with the same high quality and appearance as metal parts using electrostatic paint spray equipment.
In order to electrostatically spray paint plastic substrates, a number of technical problems must be overcome. For instance, electrostatic charges accumulate on the surface of a plastic substrate during the electrostatic spray painting process. The charges that accumulate do not dissipate as readily as with metals. This accumulation of charges reduces the potential between the spray head and the substrate, leading to weaker electrical forces on the charged paint droplets. The accumulated charges on the substrate surface also cause an opposing electrical field that repels air-borne paint particles; and the accumulated charges tend to produce a non-uniform field across the surface. These phenomena produce a self-limiting effect of yielding less paint deposition and producing less uniformity in the build-up of the paint film when compared with painting metal substrates.
In addition, some plastics have retained charges that may continue to exist for long time periods after the paint has been sprayed, making the painted surface more vulnerable to dust attraction.
As a result of these problems, it has been difficult to achieve a high quality Class "A" paint coat by electrostatic painting of plastics. The problem is particularly difficult when the objective is to apply uniform paint coats to plastic panels having complex three-dimensional shapes.
One solution to the problem has been to search for certain plastic substrate materials that will alleviate the surface charge problem and the resulting low deposition and non-uniform build-up of paint films on plastics. This approach has not proved successful to date.
Another approach has been to develop electrically conductive primers which are air-sprayed onto the plastic substrate prior to electrostatically spraying on the finished paint film. Use of a conductive primer can reduce the problems of accumulated electrostatic charges, low paint film build-up and non-uniform conductivity and film thickness. A further approach has been to add conductive materials to the molding compound, but this can degrade the physical properties of the finished part. Use of conductive polymers has also been tried, but this approach is too expensive.
In order to use such conductive primers, certain technical problems first must be overcome. There is a need for good adhesion of the primer to the plastic substrate. Special problems have been controlling the surface smoothness of the primer and achieving good adhesion to low energy substrates such as TPO (thermoplastic polyolefin) and polypropylene. The conductive primer also should have a good level of surface conductivity along with humidity insensitivity, uniformity of conductivity across the primer surface, and durability. If surface conductivity is too low, non-uniform build-up of the paint film can result. Surface conductivity, as measured in terms of "resistivity" (ohms per inch or ohms per square), should be reasonably insensitive to humidity; otherwise non-uniformities in conductivity and in the paint film build-up are produced. Other factors also can alter the uniformity of surface conductivity. When coating thickness varies as the primer is applied, it is also more difficult to achieve such uniformity.
Generally speaking, the use of conductive primers for plastic substrate panels in the automotive industry has not been successful in economically producing a Class "A" quality finish. Because of non-uniform conductivity and primer film thickness, these priming techniques have resulted in a generally poor appearance of the finished paint film. That is, a non-uniform primer, even though an undercoat in the process, can create a poor appearance of the finished exterior paint coat. It is difficult to produce a uniform paint film thickness with a primer applied by non-electrostatic air spray techniques, followed by air spraying a charged-particle paint film. Moreover, even with uniform conductivity and primer thickness, the sprayed surface can result in less than a Class "A" finish, such as an "orange peel" surface. In addition, the techniques of using conductive primers have resulted in a high scrap rate and increased production time. The current method of priming plastic parts for electrostatic paint spraying is by adding an additional step by either shipping to a separate location for priming, or priming on the paint line at the assembly plant. This amounts to high transportation and handling costs and a higher than normal scrap return rate. It also creates an additional source of volatile organic compounds. If the full car body is successfully made of plastic, the current use of a plating bath for metal parts can be eliminated from the production process.
The present invention, in one embodiment, provides a thermoformable conductive laminate that converts a non-conductive surface to a conductive surface for electrostatic painting applications. The conductive laminate overcomes the problems of non-uniform conductivity and film thickness, as well as providing a high level of conductivity uniformly across the surface of the laminate. The laminate is formed by techniques that provide a uniform conductive primer on the surface of the thermoformable laminate. The resulting laminate can be thermoformed into complex three-dimensional shapes which can then be electrostatically sprayed with a uniform paint coat after thermoforming. When used as a component in a plastic car body panel, the thermoformed laminate can be bonded to a substrate, for example, molded with thermoplastic resins, or molded with thermoset resins by various sheet molding techniques, or vacuum pressure formed and bonded to the plastic substrate. Examples of molding techniques and materials include SMC (sheet molding compound), BMC (bulk molding compound), TMC (thick molding compound), RIM (reaction injection molding), and RTM (resin transfer molding). (TMC is a trademark of Takela Chemical Industries, Ltd.) The primed part is then ready for painting directly after molding with thermoplastics or thermosets, or vacuum pressure forming. This eliminates the extra transportation and cycle time costs associated with the current off-line process of electrostatically spray painting plastic car body panels.
As a further advantage, the conductive primer retains uniformity of its conductivity throughout the thermoforming process. The primer comprises an electrically conductive uniform film with good elongation and adhesion properties. By maintaining its uniform conductivity during thermoforming, a paint coat applied to the primed conductive surface can achieve a Class "A" quality finish, even for complexly shaped panels. Less over-spray and scrap rate also are produced.
In addition to its use in the automotive industry, the invention can be used for making any contoured plastic panel in preparation for electrostatic spray painting. The thermoformable conductive laminate can be used for making doors or cabinets, or used in the electronics industry, for example, in electromagnetic shielding.